1. Field of the Invention
This invention relates to optics and, in particular, to an integrated laser imaging and spectral analysis system.
2. Related Art
Semiconductor chip manufacturers have increasingly sought to improve die yields in their production processes even with ever decreasing minimum feature sizes ("die yield" is defined as the percentage of final working dies to total attempted dies). Key to this effort is the examination of the physical features (e.g., defects) of the semiconductor wafer.
Defects are classified broadly as particulate or process flow defects. Particulate defects are undesired particles that sometimes become attached to the surface of a wafer due to contamination in the wafer processing environment. Process flow defects are differences between the actual and desired process result. The examination of particulate defects is important to determine the impact of such contamination on future process steps and ultimately on the functionality of the final die package. The examination of process flow defects is important in determining if the process is acceptable or requires furthermore recipe modifications. As minimum feature sizes continue to get smaller well into the sub-micron range, the ability to evaluate ever smaller defects on a wafer has become of paramount importance to achieving high yields.
To detect defects, instrument suppliers developed wafer scanners which scan wafers for anomalous optical sites that are characteristics of defects. In one type of wafer scanner, called a "laser scanning system", a laser beam is focused on and scanned over the surface of a test chip. Anomalous optical sites are identified by comparing the light scatter from locations on known good chips to the light scatter from the test chip. If the two light scatters are different, an anomalous optical site is detected.
In another type of wafer scanner, called a "video system", a video picture of the surface of a known good chip is taken and compared to a corresponding video picture taken of a test chip using, for example, white light imaging. For each pixel, a difference is calculated between pixel values of the known good chip image and the corresponding pixel values of the test chip image. If the difference is greater than a predetermined threshold, an anomalous optical site is identified.
As these wafer scanners were developed, the need to identify positively the defect nature (e.g., defect material, type, size, and precise location) was not appreciated. The defect nature can be used to determine the origin of the defect. The number, location and size of the defects can be used to calculate the density of defects, and, along with the defect type, the density of particular defect types can be calculated. This information can then be used to more closely monitor and/or to modify processing environment conditions and process steps in the chip production process.
As the need for more precise defect analysis has become apparent, semiconductor manufacturers have needed the ability to "revisit" defects to identify the nature, location and size of defects found by the above-described wafer scanners. This need has led to review stations that are based on laboratory microscopes with precision wafer handling stages that allow an operator to close in on and evaluate the previously detected defects. Revisiting of the defects by the review stations is done off-line from the defect detection process so as not to limit the throughput of the wafer scanners. Little engineering was done in the design of these review stations. For example, the review stations typically use off-the-shelf, visible light, research-style microscopes.
The off-the-shelf microscopes currently being used in defect review stations lack sufficient resolution to resolve sub-micron defects. Visible light confocal scanning microscopes (both white light and laser-based) that are built by modifying off-the-shelf microscopes can improve the resolution due to the inherent properties of confocal imaging which eliminates light from out of focus portions of the sample, improves the inherent point resolution characteristics, and in the case of a laser confocal, uses a single wavelength to eliminate chromatic aberrations. However, the use of such microscopes increases the risk of contamination of the semiconductor chips during the review process, since a human is relatively dirty by cleanroom standards and is in close proximity to the wafer surface when using the microscope. Furthermore, the presence of the microscope causes turbulent flow of the air near the wafer which tends to pull in nearby contaminants to the wafer.
The semiconductor industry has used scanning electron microscopes (SEMs) that provide increased resolution and perform energy dispersive X-ray (EDX) analysis. In EDX analysis, X-rays are directed toward the surface of the semiconductor chip. By measuring the wavelength spectrum of the reflected light, information can be gleaned regarding the types of material present on the wafer surface.
Unfortunately, EDX analysis requires high voltage (up to approximately 40,000 volts) SEMS. Furthermore, bombardment of the wafer surface with electrons from high voltage SEMs damages the wafer, thereby decreasing die yield.
Recently, low voltage SEMs (100-1000 volts) have seen limited use in wafer fabs for "critical dimension" measurements of line widths. However, low voltage SEMs are too slow to use except on a sample basis, and, in addition, provide no analytical (e.g., EDX) capability. Further, in both high and low voltage SEMs, the time to load samples into the SEM and pump down the load-lock chamber containing the SEM is relatively long, thereby slowing down wafer processing.
EDX technology also suffers in that only elemental information, not molecular structure information, is obtained about the bombarded surface. For example, EDX technology determines that the bombarded surface is composed of, for example, carbon. However, no information is gleaned as to whether the surface is diamond, graphite, or carbon dust. EDX technology also does not provide enough signal amplitude to analyze very light elements such as hydrogen. EDX technology is also limited to particulates of over a micron or more in size unless the particulate is a high Z material. In the industry, there is currently a need to analyze much smaller particles.
Some major semiconductor producers use systems which include both low and high voltage SEMs. However, such systems are relatively expensive, require a time consuming vacuum pump down, and still lack the ability to analyze molecular structures or sub-micron particles.
The semiconductor processing industry also uses infrared spectroscopy which operate on the principle of absorption. Since the wavelength at which a sample absorbs is a function of the sample material, the sample material is determined from the absorption spectrum.
Infrared absorption devices have a relatively large spot size due to the relatively long wavelengths of infrared light. Thus, the resolution is too imprecise to detect sub-micron defects.
One device which overcomes many of these problems is described in commonly owned U.S. Pat. No. 5,479,252 ("'252 device"), filed Jun. 17, 1993, issued Dec. 26, 1995, and entitled "laser imaging system for inspection and analysis of sub-micron particles". The '252 patent is incorporated herein by reference in its entirety. The '252 device uses confocal laser scanning microscopy to image a sample.
The '252 device has many advantages over the prior art but has no provision for spectroanalyzing the molecular structure of the sample. Thus, if spectral analysis is desired, the wafer needs to be transported to a separate spectral analysis system.
Therefore, a system which generates scanned images and reduces the amount of time between imaging and spectral analysis is desired.